Time division multiplexing (TDM) has been a preferred choice for multiplexing digital streams of data from/to multiple termination devices to/from a unique processing unit (host controller) through a serial interface. The pursued goal is in most cases hardware and wiring elimination to take advantage of the processing power of the host controller.
Simple implementations of this technique carry the following restrictions:
the master clocking of the serial link is a constant frequency, and PA1 the data rate of each terminal point is constant to avoid complex channel bandwidth management.
The repetition period is broken into equal time segments (called timeslots) and each interval is associated with a communication channel to/from a terminal device thus creating a `virtual` connection. Constant data rate is often referred as `synchronous` data transfer.
To overcome the previous mentioned restrictions, prior art systems implement elaborated control channel mechanism, rely on data buffering into FIFO memories (that introduce delays) and/or require dedicated non standard serial interface on the host processor.
There is a need for overcoming the previous mentioned restrictions allowing synchronous transfer of data with several data processing terminations over a single serial interface (data rate of each termination device is independent). This should be achieved without any complex associated control or large buffering. The serial interface on the host processor should also not require any additional nonstandard hardware.
One recent attempt to solve these problems is the Audio Codec 97 specification (AC97). The concept of having multiple codes on a single serial link and a validity tag to describe the content of each channel on the link is specified there. However the AC97 validity bit describes a static status (presence/absence) of each terminal device. Moreover every CODEC is operated at a constant sampling rate (48 Kss) and the serial link clock is also constant. Additionally the AC97 serial interface proposed implementation is not standard and relies on a dedicated AC97 controller.
There thus remains a need for an interface that provides for the transport of multiple variable rate data streams on a single serial link. Control overhead should be minimized. The interface should be operable with non-constant clocking. Finally, data delay should be minimal to accommodate real-time data.